It is known to provide optically variable devices in which arrays of microlenses focus on an object surface containing multiple sets of interleaved image elements. Each set of image elements belongs to a distinct image, so that, as a user viewing the device changes their relative angle of view, a different image becomes visible.
Image elements may be printed onto the object surface using a rotary printing process, for example gravure printing or offset printing. However, the initial portions of the image elements printed may suffer from a distortion known as feathering. This is particularly applicable to gravure printing processes.
It is desirable to produce image elements with reduced distortion due to feathering.
Security Document
As used herein, the term security document includes all types of documents and tokens of value and identification documents including, but not limited to, the following: items of currency such as banknotes and coins, credit cards, cheques, passports, identity cards, securities and share certificates, driver's licences, deeds of title, travel documents such as airline and train tickets, entrance cards and tickets, birth, death and marriage certificates, and academic transcripts.
Transparent Windows and Half Windows
As used herein the term window refers to a transparent or translucent area in the security document compared to the substantially opaque region to which printing is applied. The window may be fully transparent so that it allows the transmission of light substantially unaffected, or it may be partly transparent or translucent partially allowing the transmission of light but without allowing objects to be seen clearly through the window area.
A window area may be formed in a polymeric security document which has at least one layer of transparent polymeric material and one or more opacifying layers applied to at least one side of a transparent polymeric substrate, by omitting least one opacifying layer in the region forming the window area. If opacifying layers are applied to both sides of a transparent substrate a fully transparent window may be formed by omitting the opacifying layers on both sides of the transparent substrate in the window area.
A partly transparent or translucent area, hereinafter referred to as a “half-window”, may be formed in a polymeric security document which has opacifying layers on both sides by omitting the opacifying layers on one side only of the security document in the window area so that the “half-window” is not fully transparent, but allows some light to pass through without allowing objects to be viewed clearly through the half-window.
Alternatively, it is possible for the substrates to be formed from an substantially opaque material, such as paper or fibrous material, with an insert of transparent plastics material inserted into a cut-out, or recess in the paper or fibrous substrate to form a transparent window or a translucent half-window area.
Opacifying Layers
One or more opacifying layers may be applied to a transparent substrate to increase the opacity of the security document. An opacifying layer is such that LT<L0, where L0 is the amount of light incident on the document, and LT is the amount of light transmitted through the document. An opacifying layer may comprise any one or more of a variety of opacifying coatings. For example, the opacifying coatings may comprise a pigment, such as titanium dioxide, dispersed within a binder or carrier of heat-activated cross-linkable polymeric material. Alternatively, a substrate of transparent plastic material could be sandwiched between opacifying layers of paper or other partially or substantially opaque material to which indicia may be subsequently printed or otherwise applied.